This invention relates to a process and apparatus for the production of cement clinker and in particular to a process and apparatus for the manufacture of cement clinker utilizing a fluidized bed apparatus. The process and apparatus are particularly suited to small capacity plants and specifically to the production of cement of a light color or white cement.
Prior to the present invention, portland cement has been produced in long rotary kilns utilizing the wet or dry processes, relatively short kilns utilizing suspension preheaters, with or without precalcining systems, and in fluidized bed reactors. Most modern cement plants are designed to produce large capacities on the order of 1,000 tons per day and up to 5,000 tons per day. There are many situations where it is desirable to provide a cement clinker producing facility having a small capacity on the order of between 25 tons per day and 500 tons per day. These small capacity plants are sometimes referred to as "mini" cement plants.
Prior to the present invention, "mini" cement plants have utilized small rotary kilns with perhaps a single stage preheater and a rotary cooler. To my knowledge, a small capacity cement clinker production facility utilizing a fluidized bed apparatus and auxiliary equipment according to the present invention has not been proposed.
An efficient low capacity cement plant is desirable where cement usage is low such as in remote regions of developing countries and for the manufacture of specialty cements such as white cement.
White cement is produced from raw materials low in coloring elements such as iron, manganese and chromium, normally utilizing high grade limestone, white clay and pure silica as raw materials. White cement is used primarily for architectual applications requiring white or colored concrete.
Due to a low iron content, normally less than 0.5% Fe.sub.2 O.sub.3, the potential liquid formation within the clinkering process for white cement is low and the burnability of the raw material is very hard. A typical silica modules for white clinker is in the 4 to 6 range compared with 2.3 and 3.0 for gray cement. As is well known the silica modules is determined as follows: ##EQU1##
Due to the difficult burnability of the raw materials, the raw materials for white cement manufacture are usually ground very fine with 90% to 95% being -200 mesh being a typical grind. The grinding takes place in raw mills utilizing ceramic or alloy steel grinding media to reduce possible iron contamination of the raw material.
With white cement manufacturing apparatus, recuperative air quenching clinker coolers used in normal gray cement applications are not utilized as the clinker must be protected against oxidation of the iron content contained therein down to at least 700.degree. C. and preferably lower. Typically, water and oil sprays are used at the discharge end of the kiln to prevent oxidation after the burning zone and water quenching in drag conveyors is used after the clinker is discharged from the kiln. Some applications have used a decolorizing cooler using a CO.sub.2 or CO gas as the cooling medium. The absence of a recuperative air quenching clinker cooler tends to result in high fuel consumption for a white cement manufacturing process.
Equipment used conventionally for the production of white cement generally has short refractory life. This is due to the low potential liquid in the raw material which results in little coating in the hot zone of the conventional rotary kiln.
Fuels utilized in the manufacture of white cement are usually either natural gas or oil. Solid fuels such as coke or coal are not normally used in the manufacture of white cement due to the potential spot contamination of the material caused by ash fusion and incomplete chemical dispersion of the ash. As stated in U.S. Pat. No. 3,114,648 issued Dec. 17, 1963, free carbon in the clinker is objectionable due to the dark color it imparts to the finished cement.
Due to the difficult burnability of raw meal used for manufacturing white cement, the lack of a clinker cooler which utilizes heat recuperation and high kiln radiation due to high temperature of the kiln, fuel consumption for manufacturing white cement is typically 50% to 100% greater than that for the manufacture of gray cement clinker. It would be desirable to provide a process and apparatus capable of reducing the fuel consumption required to produce white cement, increase refractory life and permit the utilization of coal as a fuel.
It would also be desirable to provide a cement manufacturing process and apparatus which would allow reduced capital investment for a low capacity plant and be capable of utilizing solid fuel.